In the 35 years since their launch, the two Voyagers have gone where no man or machine has gone before. They flew past Jupiter, Saturn, Uranus, and Neptune, and now Voyager 1 is on course to become the first man-made object to leave our solar system and enter the interstellar medium—the space between stars. (In 2012, PopMech gave a Breakthrough Award to the Voyager team for pushing the frontiers of space exploration.)

This spring, several news organizations inaccurately reported that Voyager 1 had already left the heliosphere—the bubble of charged particles that encircles the sun and its planets. Voyager scientists quickly corrected the reports, stating that the spacecraft remained inside but had encountered some anomalies. Today, three papers published in (Science) provide a more thorough description of the nature of those anomalies. Voyager 1 is still within the solar system, they say, but it has discovered an unexpected layer in the boundary that separates our solar system from the interstellar medium.

"This is a totally new region, not a region predicted by any of our models," Edward Stone, project scientist on the Voyagers, said in a (Science) podcast.

Scientists determine just where Voyager 1 is by studying what kinds of particles strike the spacecraft. Inside the heliosheath, scientists expect Voyager 1 to be bombarded with lots of energetic particles erupting from the sun. The spacecraft should also be able to sense the direction and intensity of the sun's magnetic field. However, while the spacecraft is still inside the solar system, it shouldn't detect low-energy cosmic rays. These rays bounce around interstellar space but can't penetrate the heliosphere. Once beyond the heliosphere, the spacecraft would sense strong cosmic rays, but no particles, and the direction of the magnetic field would change.

On August 25, 2012, something dramatic happened: Voyager 1 stopped getting hit with particles and started detecting cosmic rays for the first time. But the magnetic field didn't change direction, which is what you'd expect if the spacecraft had left the solar system.

So Voyager 1 appears to be in a strange, unexpected region, and scientists aren't sure why such a strange place exists. But Stone gave one possible explanation: "It must be because somehow the solar magnetic field, which is in a huge spiral around the sun—because the sun rotates as the field is carried out, it wraps it into a huge spiral—that somehow the ends of that spiral have become connected, or at least accessible for particles inside to leave, and particles on the outside to enter. Sort of like a magnetic highway, with things inside streaming out, and particles outside streaming in."

Because it's still within the sun's magnetic field, Voyager 1 has not yet broken through into the interstellar medium. So how long until it finally does? Stone and his team just don't know how wide this new region is or how long it will take to cross. He says it could be a few months or several years for Voyager to clear it completely.

Voyager 1 is currently 11 billion miles away from Earth, or about three times the distance between Earth and Neptune. The little-spacecraft-that-could is expected to keep chugging along until 2025, when it will run out of fuel. Who knows what else it'll discover along the way, in the outer regions of our solar system and beyond. "We still have hopefully a good part of the journey left," Stone says.